As used herein, the term "pacing" refers to the delivery of a stimulation pulse to cardiac tissue, the term "antitachycardia pacing" (or ATP) refers to the delivery of any of one or more stimulation pulses that are intended to revert a tachyarrhythmia, including ventricular and/or atrial pacing, and the term "tachyarrhythmia" refers to a rapid abnormal cardiac rhythm, including ventricular fibrillation (VF), ventricular tachycardia (VT), sinus tachycardia (ST), and superventricular tachycardia (SVT); SVT includes atrial tachycardia, atrial flutter, and atrial fibrillation; a normal cardiac rhythm without tachyarrhythmia is called sinus rhythm (SR).
The medical devices that are the subject of this invention sense atrial and ventricular cardiacsevents (or beats), and derive therefrom one or more of the following intervals: between ventricular beats (RR), between atrial beats (PP), from an atrial beat to a ventricular beat (PR), and from a ventricular beat to an atrial beat (RP). They alternatively or in addition count the number of atrial beats (events) (Na) or ventricular beats (events) (Nv) in a predetermined number of cardiac cycles. As used herein, the term beat is synonymous with cardiac events, depolarization, complexes, and a heartbeat.
Arzbaecher, Bump, Jenkins, et al. PACE, Vol. 7, pp. 541-547 (1984) recognized that sensing heartbeats in a single chamber to determine when to apply ATP in that chamber, may result in inappropriate pacing and subsequent acceleration or even initiation of a tachycardia. They proposed sensing both atrial and ventricular heartbeats, and dividing intermediate-rate tachyarrhythmias into three zones: (1) When Nv&gt;&gt;Na, they classified this as VT; (2) When Na&gt;&gt;Nv, they classified this as SVT or ST; and (3) When Na approximately equals Nv, they applied one of two VT criteria: (a) acceleration of ventricular rate exceeding a preset limit, and (b) change in ventricular rate, following a premature atrial stimulus, not exceeding a preset limit.
Schuger, Jackson, Steinman and Lehmann, PACE, Vol. 11, pp. 1456-1464 (1988), proposed a criterion for distinguishing VT, consisting of stable RR and unstable PR. They further suggested averaging these PR and RR measurements over several cycle lengths to avoid falsely satisfying the VT criterion on premature ventricular contractions (PVCs).
U.S. Pat. No. 4,860,749 to Lehmann divided intermediate-rate ventricular rhythms into three zones: RR&lt;PP (equivalent to Arzbaecher's Nv&gt;&gt;Na), RR=PP (equivalent to Nv=Na) and RR&gt;PP (equivalent to Nv&lt;&lt;Na). Lehmann classified RR&lt;PP as VT. For RR&gt;PP, Lehmann applied the VT criterion: RR stable and PR unstable. For RR=PP, Lehmann applied the VT criterion: PR longer than PR in sinus rhythm.
The inventors have recognized that .the background art proposes a stability criterion for determination of pace-terminable ventricular tachyarrhythmias, where it detects pace-terminable VT when there is RR stability and PR instability. This method has as its origin the expectation that in pace-terminable VT, most ventricular beats are caused by a preceding ventricular beat, conducted by a circular pathway with stable conduction time. However, when most ventricular beats are caused by atrial beats, conducted by an atrioventricular pathway with stable conduction time, then this is not pace-terminable VT.
The background art recognizes that this criterion applies when there is not 1:1 atrioventricular association. However, when there is 1:1 association, the criterion cannot always distinguish an atrial tachyarrhythmia conducted to the ventricle from a ventricular tachyarrhythmia conducted to the atrium.
Hence, the inventors also have realized that there are deficiencies in the background art, particularly in the calculation and application of the stability criterion. In this regard, the Lehmann U.S. Pat. No. 4,860,749 does not disclose which atrial beat or beats to take into account when calculating the PR interval. When Na&gt;Nv, more than one atrial beat can be detected per ventricular cycle. If more than one PR interval per cycle is taken into account in an averaging calculation of stability, this will make the PR interval appear unstable, even though atrioventricular conduction with fixed block and very stable PR interval may be in progress.
In addition, the aforementioned background art only applies the stability criterion when Na&gt;Nv. As recognized by the inventors, if one considers the onset of ST with Na=Nv, the RR interval decreases (the rate accelerates), but PR remains constant. Thus, as the inventors have appreciated as discussed below, the stability criterion could be applied to classify this, correctly, as not VT.
The Lehmann U.S. Pat. No. 4,860,749 does not disclose how to calculate PR instability or RR stability. The Schuger publication (with Lehmann) does suggest to use averages over several cardiac cycles. However, this technique is grossly affected by a single premature beat, a burst of electrical noise, or by a single heartbeat which is not sensed.
The inventors also have recognized that the stability criterion could be applied to detecting pace-terminable tachyarrhythmias in the atrium as well as the ventricle.